ubootTools: fix build by fixing -idirafter ordering

`ubootTools` build broke after NixOS#210004
where we started dropping default libc include path and switched to
`-idirafter` way of specifying libc headers.

Unfortunately the way it's implemented it injects -idirafter after
user's flags, not before. That allows users to inject their paths before
libc include paths, not after (as it would notmally happen).

The change works it around for u-boot by pulling -idirafter libc flags
before user's flags.

pkgs/misc/uboot/default.nix

@@ -92,6 +92,14 @@ let
 
     passAsFile = [ "extraConfig" ];
 
+    # Workaround '-idirafter' ordering bug in staging-next:
+    #   https://github.com/NixOS/nixpkgs/pull/210004
+    # where libc '-idirafter' gets added after user's idirafter and
+    # breaks
+    preConfigure = ''
+      export NIX_CFLAGS_COMPILE_BEFORE_${lib.replaceStrings ["-" "."] ["_" "_"] stdenv.hostPlatform.config}=$(< ${stdenv.cc}/nix-support/libc-cflags)
+    '';
+
     configurePhase = ''
       runHook preConfigure